1. Field of the Invention
This invention relates to wireless transmission systems and, more particularly, to the use of noise as an alerting signal to indicate a poor transmission path in such a wireless transmission system.
2. Description of the Prior Art
Wireless systems are known for short-range transmission which utilize ratio signals, ultrasonic signals and light signals to transmit voice signals between a wirelesss portable transmitter and a stationary receiver. Ultrasonic and radio transmissions have a distinct disadvantage for telephony, however, due to the lack of privacy engendered by the range and ability of these signals to pass through many common buildings. Signals in the light frequency range, on the other hand, are stopped or severely attenuated by normal building structures and afford a high degree of privacy. In particular, signals in the infrared band are not visible, provide a line-of-sight transmission and are therefore highly suitable for cordless or hands-free telephony.
The highly directional nature of infrared transmission, however, poses a serious problem in maintaining an adequate communication path between a portable transmitter and a stationary receiver. While a large number of spatially distributed receivers alleviates this problem, it does so at a significant cost and with a residual possibility that an adequate transmission path may not exist in spite of the arrangement.